An active device of that type is described in document EP 0 574 574 B1, for example.
Known devices of that type generally make use of a permanent magnet. Permanent magnets are fragile and sensitive to temperature. Their action is thus affected by variations in temperature, which variations are frequent in the vicinity of a motor vehicle engine. Furthermore, the cost of such magnets is high because of the rare materials that are used and because of the method used for making such magnets.